This invention relates generally to a temperature to frequency converter and more particularly, but not by way of limitation, to such a converter that provides a constant amplitude output signal having a frequency inversely proportional to sensed temperature.
Downhole static and circulating temperatures at various depths are typically important characteristics to know when drilling or completing an oil or gas well. For example, an operator can use knowledge of these temperatures to help determine retarder requirement for cement designed to be used in the well. That is, a cement composition designed for the right temperature will help minimize problems associated with the setting time of the cement. For example, a cement designed more accurately for thickening time and compressive strength development will help improve cement bonding and drillout times and minimize gas migration. Measurements of such temperatures can also be useful for stimulation purposes, for example.
I am aware of a downhole tool that can measure downhole static and circulating temperatures. This tool has a temperature to frequency converter that converts sensed temperature to an electrical signal having a frequency directly proportional to the sensed temperature (i.e., as temperature increases, frequency increases; and as temperature decreases, frequency decreases). The temperature sensitive element is a crystal cut so that an electrical characteristic (namely, resonance) of the crystal changes in response to temperature. Change of this characteristic changes the output frequency of an oscillator circuit in which the crystal is connected. Although this converter is functional, it has shortcomings.
One shortcoming of the crystal-based temperature to frequency converter of the aforementioned downhole tool is that the crystal is relatively fragile and is therefore susceptible to being broken in the rough environment of an oil or gas well. Another drawback of this type of converter is that the electrical conductors, or leads, connecting the crystal into the oscillator circuit have to be electrically and mechanically stabilized so that there is no capacitance change. That is, the leads between the temperature sensitive crystal and the oscillator circuit can affect the frequency response of the oscillator. Without stabilization, the output of this type of temperature to frequency converter would not be reliable. Still another shortcoming of this type of converter is that the crystal is relatively large; therefore, this size must be accommodated in the downhole tool where space is typically limited. The crystal of this type of converter also is typically limited to sensing temperatures up to about 150.degree. C.
In view of the shortcomings of the crystal-based temperature to frequency converter, there is the need for an improved converter which does not have these shortcomings and yet can be used in an otherwise conventional downhole tool to measure static and circulating temperatures. Such an improved temperature to frequency converter should also be useful in order tools and other applications.